It is conventional, with TEM schemes, to use an airborne system in which a transmitter loop is located with or within an aircraft, and the associated receiver sensor is also carried by the same aircraft.
Such technique is very efficient in so far that a very large area can be covered in a relatively short time but because the size of the transmitter loop is inherently limited there is a limitation as to the effective depth that can be interrogated with such an arrangement.
On the other hand, by using a ground based transmission antenna, the size of the antenna can be very large indeed but it will inherently take significant time to move throughout the area to take appropriate readings.
Currently MMR and MIP surveys are conducted on the ground using a ground based transmitter and ground based receiver. As mentioned above, the main limitation of this approach is that it is slow and requires ground access to all portions of the survey area. This makes these survey methods relatively expensive.
One form of this invention can comprise a method of interrogating volume of material beneath the ground which comprises locating a ground based loop or grounded conductor across any area to be surveyed, effecting transient electromagnetic pulses into the ground based loop or grounded conductor, and detecting the results of such transient electromagnetic pulses from an airborne vehicle.
Such an arrangement has been found to provide very significant advantages over that which has gone on hitherto.
Nobody seems to have hitherto realized that by separating the transmitter loop from the airborne vehicle the loop can be made very large indeed can be located conveniently and extensively in a close position relative to the ground and significant time can be saved. Having a receiver carried by an airborne vehicle, such as either a helicopter or a fixed wing aeroplane, the gathering of information can be quickly achieved. The receiver itself can be highly sensitive and the detector can be carried either in an aeroplane or other airborne vehicle or preferably, in a bird towed behind the airborne vehicle. The receiving sensor can feed the signals into a memory means within the vehicle. All of these features in combination provide very significant advantages.
The advantages provided by this arrangement include an increased signal to noise ratio and, furthermore, the effects of the electromagnetic transient pulse can be measured out to longer time intervals which have the advantage of enabling a deeper depth to be interrogated.
One of the difficulties in using a ground based transmitter and an airborne sensor is that the signal power must be very high in order to give a sufficient signal over a loop that necessarily becomes very large. There has also been a major difficulty in being able to effect a sufficiently large power supply which, at the same time, can provide a signal of sufficient quality for such purposes.
A second problem has been that in order to effect useful readings, the sensor or the receiver must be able to be extremely and accurately synchronized or at least have access to synchronizing signals from the ground loop.
It is now possible to achieve a sufficiently accurate synchronization calibration which will hold at least over a few hours. The calibration is achieved by having very highly accurate crystal control oscillators which are first brought into an identical frequency oscillation and, then, prior to being separated, are calibrated to a sufficient extent that the receiver, which thereafter will be physically separated from the ground loop or the transmitter of the ground loop, will be able to predict with very great accuracy the start time with respect to each transient electromagnetic pulse. At the same time, the receiver will also be able to predict with great accuracy the cutoff time so that appropriate measurements of the received signals can be made from that cutoff time. This method produces accurate readings appropriate to the pathway that the signal and its environs has travelled.
It will be appreciated that the invention does not concern the specific details of the TEM, MMR or MIP analysis as applied to collected data but to the application of those techniques to data gained using a ground based transmitter and an airborne receiver. Details of TEM, MMR and MIP analysis are well known to skilled addressees as typified by:
Transient Electromagnetic Method (TEM):
Sherif R. E. 1984 "Encyclopedic Dictionary of Exploration Geophysics", Published SEG (USA). PA1 Spies B. R. 1980 "TEM in Australian Conditions", Research in Exploration Geophysics, Published Macquarie University. PA1 Spies B. R. 1980 "Interpretation and design of time-domain EM surveys in areas of conductive overburden", D. W. Emerson (ed) Bulletin of Aust. Soc. Expl. Geophys. Vol. 11. pp13-139. PA1 Nabighian M. N. 1979 "Quasi-static transient response of a conductive half-space:--An approximate representation", Geophysis Vol. 44. pp 1700-1705. PA1 Sherrif, R. E. 1984 "Encyclopedic Dictionary of Exploration Geophysics", Published SEG (USA). PA1 Seigel, H. O. 1974 "The Magnetic Induced Polarisation Method", Geophysics Vol. 39. pp321 339. PA1 Howland-Rose, A. W.; Linford, J. G.; Pitcher, D. H.; and Seigel H. O. 1980 "Some recent magnetic induced polarisation developments", Geophysics Vol. 45. parts I and II pp37-74. PA1 locating a transmitter means over, in close proximity to or in contact with, a surface area of ground encompassing and beneath which lies the volume of material; PA1 generating with generator means transient electromagnetic pulses within the transmitter means; PA1 flying over the said area of ground with an airborne vehicle aboard which is a detector means; PA1 detecting with the detector means electromagnetic fields resultant of such transient electromagnetic pulses; PA1 forming a received signal dependant upon the magnitude and temporal characteristics of the detected electromagnetic fields; and, PA1 processing the received signal to determine characteristics of the volume of material. PA1 locating a loop antenna over, in close proximity to or in contact with, a surface area of ground encompassing and beneath which lies the volume of material or locating a grounded conductor or conductors in a surface area of ground beneath which lies the volume of material; PA1 generating with generator means transient electromagnetic pulses within the loop antenna or the grounded conductor or conductors; PA1 flying over the said area of ground with an airborne vehicle aboard which is a detector means; PA1 detecting with the detector means electromagnetic fields resultant of such transient electromagnetic pulses; PA1 forming a received signal dependant upon the magnitude and temporal characteristics of the detected electromagnetic fields; and, PA1 processing the received signal to determine characteristics of the volume of material. PA1 forming a digital signal by repetitively digitally sampling the received signal; and, PA1 storing the digital signal in a suitable memory means. PA1 a transmitter means located over, in close proximity to or in contact with, an area of ground encompassing and beneath which lies the volume of material; PA1 a generator means adapted to generate transient electromagnetic pulses within the transmitter means; PA1 a vehicle means adapted to fly over the said area of ground aboard which is a detector means; PA1 a detector means adapted to detect electromagnetic fields resultant of such transient electromagnetic pulses; PA1 a receiver means adapted to form a received signal dependent upon the magnitude and temporal characteristics of the detected electromagnetic fields; and, PA1 a processor means adapted to provide an output indicative of characteristics of the volume of material. PA1 a loop antenna located over, in close proximity to or in contact with, an area of ground encompassing and beneath which lies the volume of material or a grounded conductor or conductors located in an area of ground beneath which lies the volume of material; PA1 a generator means adapted to generate transient electromagnetic pulses within the loop antenna or the grounded conductor or conductors; PA1 a vehicle means adapted to fly over the said area of ground aboard which is a detector means; PA1 a detector means adapted to detect electromagnetic fields resultant of such transient electromagnetic pulses; PA1 a receiver means adapted to form a received signal dependant upon the magnitude and temporal characteristics of the detected electromagnetic fields; and, PA1 a processor means adapted to provide an output indicative of characteristics of the volume of material.
Magnetic Induced Polarisation Method (MIP) and Magnetometric Resistivity Method (MMR):